That’s the bleed nipple circled by the spring, there’s one on each single or twin piston caliper, specific calipers (BREMBO) have two.
Place a ring spanner on the bleed nipple then connect a length of hose leading to a suitable waste container.
With the help of a special friend, have them press down on the brake pedal.
Turn the bleed nipple (anti clockwise) out about a quarter of a turn and brake fluid should come out. Tighten up the bleed nipple and tell your special friend to release the brake pedal.
Do this until the brake reservoir is almost empty (don’t go beyond otherwise you’ll trap air in the system) then top it right up with DOT 4 fluid. Bleed each caliper about 250ml whilst keeping the reservoir topped up.
Once that’s done check the pedal goes firm when pressed before reaching the end of it’s travel and doesn’t feel spongy, if it doesn’t feel firm then you may have air in the system that needs bleeding.
